Selectable valve assembly for a vehicle transmission

ABSTRACT

A hydraulic valve assembly for controlling a clutch in a vehicle transmission. The valve assembly includes a housing with a first port for connection to a source of pressurized hydraulic fluid, a displaceable sealing piston disposed within the housing, a valve spring for displacing the sealing piston, a snap-spring having first and second equilibrium positions, and an overstroke spring. In the first equilibrium position a spring force displaces the sealing piston to uncover the first port to enable fluid flow through the valve, and in the second equilibrium position the valve spring displaces the sealing piston to block the first port to block fluid flow through the valve. Moving from the first equilibrium position to the second equilibrium position includes displacing the overstroke spring.

FIELD

The invention relates generally to a hydraulic valve, and morespecifically to a selectable hydraulic valve assembly for a vehicletransmission operated by hydraulic pressure.

BACKGROUND

Hydraulic valves with snap-springs are known. One example is shown inU.S. Pat. No. 4,240,458 assigned to Harper-Wyman Company. It is alsoknown to use a remote accumulator to decrease shift times in a vehicletransmission. One example of such an accumulator is shown in U.S. Pat.No. 6,162,147 assigned to General Motors Corporation.

BRIEF SUMMARY

Example aspects broadly comprise a hydraulic valve assembly forcontrolling a clutch in a vehicle transmission. The valve assemblyincludes a housing with a first port for connection to a source ofpressurized hydraulic fluid, a displaceable sealing piston disposedwithin the housing, a valve spring for displacing the sealing piston, asnap-spring having first and second equilibrium positions, and anoverstroke spring. In the first equilibrium position, a spring forcedisplaces the sealing piston to uncover the first port to enable fluidflow through the valve, and in the second equilibrium position, thevalve spring displaces the sealing piston to block the first port toblock fluid flow through the valve. Moving from the first equilibriumposition to the second equilibrium position includes displacing theoverstroke spring.

In an example embodiment, the spring force is generated by thesnap-spring. In an example embodiment, the valve assembly includes avalve release spring and the spring force is generated by the valverelease spring. In an example embodiment, the overstroke spring is aclutch release spring.

In some example embodiments, the snap-spring is displaceable to displacethe overstroke spring. The snap-spring is arranged to displace theoverstroke spring to move from the first equilibrium position to thesecond equilibrium position when acted upon by pressurized hydraulicfluid having a pressure greater than a normal operating pressure for theclutch. In an example embodiment, the snap-spring is displaceable todisplace the overstroke spring in response to an increase in hydraulicpressure on the snap-spring. In some example embodiments, the valveassembly includes an overstroke piston and the overstroke pistondisplaces the overstroke spring. The overstroke piston is arranged todisplace the overstroke spring to move the snap-spring from the firstequilibrium position to the second equilibrium position in response tohydraulic fluid pressure greater than a normal operating pressure forthe clutch. In some example embodiments, the valve assembly includes anopen piston and moving from the second equilibrium position to the firstequilibrium position includes displacing the open piston. The openpiston is arranged to move the snap-spring from the second equilibriumposition to the first equilibrium position in response to hydraulicpressure less than or equal to a normal operating pressure for theclutch.

Other example aspects broadly comprise a hydraulic valve assembly forcontrolling a clutch in a vehicle transmission including a housing witha first port for connection to a source of pressurized hydraulic fluid,a displaceable sealing piston disposed within the housing, a valvespring for displacing the sealing piston, a snap-spring having first andsecond equilibrium positions, and an overstroke spring. In the firstequilibrium position the snap spring holds the sealing piston in an openposition to enable fluid flow through the valve, and in the secondequilibrium position the snap spring is displaced away from the sealingpiston and the valve spring holds the sealing piston in a closedposition to block fluid flow through the valve. Moving from the firstequilibrium position to the second equilibrium position includesdisplacing the overstroke spring.

Other example aspects broadly comprise a clutch for vehicle transmissionincluding at least one clutch plate, a piston plate displaceable forengaging and disengaging the at least one clutch plate to form a torquepath through the at least one clutch plate, a chamber, partially formedby the piston plate for receiving and discharging fluid to displace thepiston plate, and a snap-spring displaceable between first and secondequilibrium positions. The clutch also includes a hydraulic valve havinga housing with a first port for connection to a source of pressurizedhydraulic fluid and a second port in fluid communication with thechamber, a displaceable valve piston, and a valve spring. In the firstequilibrium position, the snap-spring holds the valve piston in a openposition to enable fluid flow from the source of pressurized hydraulicfluid to the chamber via the first and second ports, and in the secondequilibrium position, the snap-spring holds the valve piston in a closedposition compressing the valve spring.

In some example embodiments, the clutch includes a displaceable sealingpiston and, during an engine idle mode, hydraulic pressure is withdrawnfrom the first port and the valve spring displaces the sealing piston toblock the second port and trap hydraulic fluid in the chamber. In someexample embodiments, the clutch includes a valve release spring and thevalve release spring displaces the sealing piston when the snap-springis in the first equilibrium position. In an example embodiment, thevalve spring generates an apply force, the valve release springgenerates a release force, and the apply force is greater than therelease force when the valve spring is compressed by the valve piston.

In some example embodiments, the clutch includes an overstroke springand moving from the first equilibrium position to the second equilibriumposition includes displacing the overstroke spring. In an exampleembodiment, the clutch includes an overstroke piston, the pressurizedhydraulic fluid displaces the overstroke piston, and the overstrokepiston displaces the overstroke spring. A pressure of the pressurizedfluid is greater than a normal operating pressure for the clutch whenthe overstroke spring is displaced.

In an example embodiment, the clutch includes a chamber for receivingand discharging fluid to displace the piston plate and the overstrokepiston displaces the overstroke spring in response to hydraulic pressurein the chamber greater than a normal operating pressure for the chamber.In some example embodiments, the clutch includes an open piston andmoving from the second equilibrium position to the first equilibriumposition includes displacing the open piston. In an example embodiment,the pressurized hydraulic fluid displaces the open piston, and apressure of the pressurized hydraulic fluid is less than or equal to anormal operating pressure for the clutch when the open piston isdisplaced.

Other example embodiments broadly comprise a hydraulic valve assemblyfor a vehicle transmission including a housing, a sealing pistondisposed within the housing and displaceable to open and close thevalve, a snap-spring having first and second equilibrium positions, avalve spring, and a displaceable overstroke spring. In the firstequilibrium position the snap-spring displaces the sealing piston tohold the valve in an open position, and in the second equilibriumposition the valve spring displaces the sealing piston to hold the valvein a closed position. Transitioning from the first equilibrium positionto the second equilibrium position includes displacing the overstrokespring.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature and mode of operation of the present invention will now bemore fully described in the following detailed description of theinvention taken with the accompanying drawing figures, in which:

FIG. 1A is a perspective view of a cylindrical coordinate systemdemonstrating spatial terminology used in the present application;

FIG. 1B is a perspective view of an object in the cylindrical coordinatesystem of FIG. 1A demonstrating spatial terminology used in the presentapplication;

FIG. 2 is a cross sectional view of a hydraulic valve shown in a normaloperating mode according to an example aspect;

FIG. 3 is a cross-sectional view of the valve of FIG. 2 shown in atransition mode;

FIG. 4 is a cross-sectional view of the valve of FIG. 2 shown in aclosed mode;

FIG. 5 is a cross-sectional view of a valve assembly according to anexample aspect;

FIG. 6 is a cross sectional view of a hydraulic valve shown in an openmode according to an example aspect;

FIG. 7 is a cross-sectional view of the valve of FIG. 6 shown in aclutch apply mode;

FIG. 7A is a detail view of a portion of the valve of FIG. 6circumferentially offset from the portion of the valve depicted in FIG.7, shown in a clutch apply mode;

FIG. 8 is a cross-sectional view of the valve of FIG. 6 shown in atransition mode;

FIG. 8A is a detail view of a portion of the valve of FIG. 6circumferentially offset from the portion of the valve depicted in FIG.8, shown in a transition mode;

FIG. 9 is a cross-sectional view of the valve of FIG. 6 shown in anidle-stop mode;

FIG. 9A is a detail view of a portion of the valve of FIG. 6circumferentially offset from the portion of the valve depicted in FIG.9, shown in an idle-stop mode;

FIG. 10 is a cross-sectional view of the valve of FIG. 6 shown in are-open mode;

FIG. 10A is a detail view of a portion of the valve of FIG. 6circumferentially offset from the portion of the valve depicted in FIG.10, shown in a re-open mode.

DETAILED DESCRIPTION

At the outset, it should be appreciated that like drawing numbersappearing in different drawing views identify identical, or functionallysimilar, structural elements. Furthermore, it is understood that thisinvention is not limited only to the particular embodiments,methodology, materials and modifications described herein, and as suchmay, of course, vary. It is also understood that the terminology usedherein is for the purpose of describing particular aspects only, and isnot intended to limit the scope of the present invention, which islimited only by the appended claims.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood to one of ordinary skill inthe art to which this invention belongs. Although any methods, devicesor materials similar or equivalent to those described herein can be usedin the practice or testing of the invention, the following examplemethods, devices, and materials are now described.

FIG. 1A is a perspective view of cylindrical coordinate system 80demonstrating spatial terminology used in the present application. Thepresent invention is at least partially described within the context ofa cylindrical coordinate system. System 80 has a longitudinal axis 81,used as the reference for the directional and spatial terms that follow.The adjectives “axial,” “radial,” and “circumferential” are with respectto an orientation parallel to axis 81, radius 82 (which is orthogonal toaxis 81), and circumference 83, respectively. The adjectives “axial,”“radial” and “circumferential” also are regarding orientation parallelto respective planes. To clarify the disposition of the various planes,objects 84, 85, and 86 are used. Surface 87 of object 84 forms an axialplane. That is, axis 81 forms a line along the surface. Surface 88 ofobject 85 forms a radial plane. That is, radius 82 forms a line alongthe surface. Surface 89 of object 86 forms a circumferential plane. Thatis, circumference 83 forms a line along the surface. As a furtherexample, axial movement or disposition is parallel to axis 81, radialmovement or disposition is parallel to radius 82, and circumferentialmovement or disposition is parallel to circumference 83. Rotation iswith respect to axis 81.

The adverbs “axially,” “radially,” and “circumferentially” are withrespect to an orientation parallel to axis 81, radius 82, orcircumference 83, respectively. The adverbs “axially,” “radially,” and“circumferentially” also are regarding orientation parallel torespective planes.

FIG. 1B is a perspective view of object 90 in cylindrical coordinatesystem 80 of FIG. 1A demonstrating spatial terminology used in thepresent application. Cylindrical object 90 is representative of acylindrical object in a cylindrical coordinate system and is notintended to limit the present invention in any manner. Object 90includes axial surface 91, radial surface 92, and circumferentialsurface 93. Surface 91 is part of an axial plane, surface 92 is part ofa radial plane, and surface 93 is part of a circumferential plane.

The following description is made with reference to FIG. 2. FIG. 2 is across sectional view of hydraulic valve 100 shown in a normal operatingmode according to an example aspect. Valve 100 may be a hydraulic valvefor a vehicle transmission. That is, valve 100 may be designed tocontrol flow of hydraulic fluid in a vehicle transmission as describedinfra. Valve 100 may control flow of hydraulic fluid to a clutch in aplanetary automatic transmission, for example.

Valve 100 includes housing 102, snap-spring 104 and overstroke spring106. By snap-spring, or snap-disk, we mean a bi-stable spring or aspring that can be “snapped through” to change its equilibrium position.By equilibrium position we mean a position where the spring remainsuntil acted on by an outside influence. Snap-spring 104 has at least twoequilibrium positions. For example, spring 104 may include concave andconvex equilibrium positions. Overstroke spring 106 may be a bellevillewasher, for example. Snap-spring 104 and overstroke spring 106 aredisposed within housing 102.

Snap-spring 104 may be sealed to housing 102 by seal 108. Seal 108 maybe a rubber seal or a teflon seal, for example. In some embodiments (notshown), spring 104 is sealed to housing 102 by a tight fit between thespring and housing. Overstroke spring 106 is arranged to act on spring104. That is, outer diameter 110 of spring 106 abuts spring 104 at outerdiameter portion 112. Inner diameter 114 of spring 106 abuts housing102. Overstroke spring 106 urges outer portion 112 of snap-spring 104 indirection of arrow 116.

Valve 100 may include internal housing 118 disposed within housing 102.In some embodiments (not shown), relevant features of internal housing118 are integrated into housing 102. Housing 118 is sealed to housing102 by seal 120. Seal 120 may be an o-ring seal, for example. Internalhousing 118 is axially fixed to housing 102 through interaction of cap122. That is, cap 122 is attached to internal housing 118 at threadedportion 124, and cap 122 is attached to housing 102 at threaded portion126. Cap 122 is sealed to housing 102 by seal 128.

Sealing piston 130 is sealed to internal housing 118 at areas 131. Areas131 are depicted as a close fit between housing 118 and piston 130,although other embodiments (not shown) may include a sealing elementbetween piston 130 and housing 118 at areas 131. Piston 130 includesangled face 132 arranged to interface with corner 134 of housing 118.That is, piston 130 and housing 118 can be sealingly engaged by contactbetween face 132 and corner 134. Valve spring 136 urges piston 130towards corner 134 of housing 118. Spring 136 is axially retained by cap122. Shaft 138 is disposed axially between inner portion 139 of spring104 and piston 130, and sealed to housing 102 by seal 140. Seal 140 maybe a low friction seal so that shaft 138 slides easily. Seal 140 may bea teflon seal, for example. In the normal operating mode shown in FIG.2, force of spring 104 is greater than force of spring 136 so that shaft138 axially displaces piston 130 in direction of arrow 142 and away fromcorner 134. That is, snap-spring 104 displaces sealing piston 130 awayfrom sealing engagement with housing 102 when valve 100 is in the openposition.

Valve 100 includes pressure ports 144, 146, and 148 disposed in housing102. Pressure ports 144 and 146 may be connected to a pressure sourceand port 148 may be connected to a clutch pack. Ports 144 and 146 may beconnected to a transmission pump, for example. Internal housing 118includes port 150 in fluid communication with port 148 of housing 102.In some embodiments where housing 118 is integral to housing 102, ports148 and 150 may be the same port. Valve 100 further includes chambers152, 154, and 155 in fluid communication with respective ports 144, 146,and 148. Pressure chamber 152 is at least partially defined by housing102 and snap-spring 104. Pressure chamber 154 is at least partiallydefined by housing 102 and sealing piston 130. Chamber 156 is fluidlyconnected to a sump (not shown) through port 158 in cap 122 to drainfluid leaking past sealed areas 131. Exhaust chamber 159 is connected toa sump (not shown) through port 160.

Operation of valve 100 will now be described. During normal operation asshown in FIG. 2, valve 100 is open. Pressurized hydraulic oil entersvalve 100 through ports 144 and 146. Pressure in chamber 152 indicatedby arrows 162 acts on snap spring 104. So long as pressure in chamber152 is below a predetermined limit, spring 104 maintains the positionshown in FIG. 2. Pressure in chamber 154 and force of spring 104 actingon shaft 138 displace piston 130 so that ports 146 and 148 are in fluidcommunication. That is, flow introduced in port 146 flows throughchamber 154 and past piston 130 to port 148. Alternatively stated, flowfrom a transmission pump can proceed from port 146 through valve 100 andout port 148 to a piston plate chamber for a clutch pack.

The following description is made with reference to FIGS. 2-4. FIG. 3 isa cross-sectional view of valve 100 of FIG. 2 shown in a transitionmode. FIG. 4 is a cross-sectional view of valve 100 of FIG. 2 shown in aclosed mode. Increased, overstroke pressure in chamber 152 indicated bybold arrows 164 acting on snap spring 104 axially displaces outerportion 112 of spring 104 urging overstroke spring 106 in direction ofarrow 142. It should be noted that the increased pressure is greaterthan a normal operating pressure for the clutch. By normal operatingpressure, we mean the pressure required to operate the transmission atthe required torque capacity. For example, the clutch requires a definedpressure in the piston plate chamber to clamp the clutch pack andtransmit required torque. The pressure to transmit the rated torque isthe normal operating pressure. Axial deflection of inner portion 139 ofthe snap-spring is restricted by contact with shaft 138, piston 130 andcap 122. Once the input pressure exceeds a predetermined value,snap-spring 104 “snaps through” to the position shown in FIG. 3. Thepredetermined value can be controlled in part by the force of overstrokespring 106.

As can be seen in FIG. 3, outer portion 112 of spring 104 has shifted indirection of arrow 142 displacing seal 108 and outer portion 110, andcompressing overstroke spring 106. That is, snap-spring 104 is arrangedto displace overstroke spring 106 to move from the first equilibriumposition to the second equilibrium position when acted upon bypressurized hydraulic fluid having a pressure greater than a normaloperating pressure for the clutch. Otherwise stated, displacement ofoverstroke spring 106 by spring 104 in response to an increase inhydraulic pressure permits axial motion of outer portion 110 anddisplacement of spring 104 from the first equilibrium position to thesecond equilibrium position. Axial force of spring 136 in direction ofarrow 116 is less than axial force in direction 142 from pressure inchamber 152. Therefore, flow from port 146 to port 148 is unrestrictedbecause angled face 132 remains displaced from corner 134.

Once pressure is removed from port 144 and chamber 152 (see FIG. 4),axial force of spring 136 urges piston 130 and shaft 138 in direction ofarrow 116. Shaft 138 urges inner portion 139 of spring 104 in directionof arrow 116. Likewise, overstroke spring 106 urges outer portion 112 ofspring 104 and seal 108 in direction of arrow 116 until seal 108contacts housing 102. Remaining pressure in chamber 155 acting on area166 of piston 130 urges piston in direction of arrow 142 until force ofvalve spring 136 overcomes the pressure force. Therefore, valve spring136 and area 166 can be adjusted to provide an appropriate pressureforce for chamber 155.

Axial motion of spring 104 in direction of arrow 116 allows axial motionof piston 130 in direction of arrow 116 until face 132 contacts corner134 restricting flow of fluid between ports 146 and 148, closing valve100. Otherwise stated, input pressure acting on snap-spring 104displaces overstroke spring 106 and adjusts valve 100 into a closedposition when the input pressure is released. That is, valve spring 136displaces sealing piston 130 into sealing engagement housing 102 whenvalve 100 is in the closed position.

Therefore, pressure is maintained in chamber 155, port 148 and thepiston plate chamber for the connected clutch pack, for example, evenwhen the pressure source (not shown) is not providing pressure. Apre-pressurized, or pre-charged, clutch pack, described above, reactsfaster during a launch event when the vehicle engine has been turnedoff. This functionality is important in start-stop vehicle strategiesfor improved fuel economy.

Valve 100 can be easily reset by the next normal pressure event. Forexample, after the stop event, pressure enters valve 100 through ports144 and 146 axially displacing inner portion 139 of spring 104. Itshould be noted that the pressure is less than the overstroke pressurerequired to close the valve and less than or equal to the normalpressure. Spring 104 displaces shaft 138 and piston 130, restoring fluidcommunication between ports 146 and 148 to engage and disengage theclutch pack. Axial displacement of outer portion 112 of spring 104 isrestricted by contract with outer diameter 110 of overstroke spring 106,so spring 104 “snaps through” to the position shown in FIG. 2. Valve 100remains in this condition with fluid communication between ports 146 and148 until the next high pressure, overstroke event.

The following description is made with reference to FIG. 5. FIG. 5 is across-sectional view of valve assembly 200 according to an exampleembodiment. In general, the above description and operation of valveassembly 100 applies to valve assembly 200. Most 2XX referencescorrespond with 1XX references described supra. For the sake of brevity,only the differences between valves 100 and 200 are now described.

Valve 200 includes close piston 268 and open piston 270, in fluidcommunication with respective ports 272 and 274. Ports 272 and 274 areconnected to a pressure source. Spring 204 is not sealed to housing 202.Normal operation of valve 200 is similar to valve 100. Force of spring236 is overcome by forces of spring 204 and pressure acting on openpiston 270, axially displacing sealing piston 230 away from housing 218and opening valve 200.

When pressure in ports 272 and 274 is increased beyond the normaloperating pressure to an overstroke pressure, overstroke piston 268axially displaces outer portion 212 of snap-spring 204, compressingoverstroke spring 206. Axial deflection of inner portion 239 isrestricted by contact with shaft 238, piston 230 and cap 222. Once theinput pressure exceeds a predetermined value, snap-spring 204 “snapsthrough” as described supra. After pressure is removed from ports 246,268, and 274, valve 200 closes, preventing leakage of fluid in port 248and the clutch pack (not shown).

As described for valve 100, valve 200 can be easily reset by the nextpressure event. For example, after the stop event, pressure enters valve100 through ports 246, 272, and 274 axially displacing open piston 270and inner portion 239 of spring 204. It should be noted that thepressure is less than the overstroke pressure required to close thevalve and less than or equal to the normal operating pressure. Spring204 displaces shaft 238 and piston 230, restoring fluid communicationbetween ports 246 and 248 to engage and disengage the clutch pack. Axialdisplacement of outer portion 212 of spring 204 is restricted bycontract with outer diameter 210 of overstroke spring 206, so spring 204“snaps through” as described supra. Valve 200 remains in this conditionwith fluid communication between ports 246 and 248 until the next highpressure event.

The following description is made with reference to FIG. 6. FIG. 6 is across sectional view of hydraulic valve 300 shown in an open modeaccording to an example aspect. Valve 300 is shown disposed withintransmission 400. That is, valve 300 may be designed to control flow ofhydraulic fluid in vehicle transmission 400 as described infra. In theexample embodiment shown, for example, valve 300 controls a flow ofhydraulic fluid to clutch 402 in planetary automatic transmission 400.

Valve 300 includes housing 302, snap-spring 304 and overstroke spring306. By snap-spring, or snap-disk, we mean a bi-stable spring or aspring that can be “snapped through” to change its equilibrium position.Spring 304 operates in convex and concave positions as described infra.Overstroke spring 306 may be a belleville washer, for example.Overstroke spring 306 is preloaded between housing 302 and snap ring307, as indicated by phantom line 306A. That is, spring 306 iscompressed against piston 302 when snap ring 307 is installed. In theembodiment shown in FIG. 6, housing 302 comprises a clutch piston. Thatis, a portion of housing 302 is also a piston for transmission clutch402.

Overstroke spring 306 is arranged to act on spring 304. That is, innerdiameter 310 of spring 306 is engaged with spring 304 at outer diameterportion 312. Outer diameter 314 of spring 306 is engaged with snap ring307 disposed in housing 302, as described supra. Outer diameter portion312 is retained between spring 306 and pivot plate 315. Overstrokespring 306 urges outer portion 312 of snap-spring 304 in direction ofarrow 316. Overstroke piston 317 is arranged to displace plate 315 andouter diameter portion 312 of spring 304, as described infra. Piston 317is sealed to housing 302 by seals 319 and 321.

Valve 300 may include internal housing 318. In some embodiments (notshown), relevant features of internal housing 318 are integrated intohousing 302. Housing 318 is sealed to housing 302 by seal 320. Seal 320may be an o-ring seal or a teflon seal, for example. Internal housing318 is axially fixed to the transmission housing 404. Housing 404 issealed to housing 302 by seal 328.

Sealing piston, or poppet, 330 is sealed to internal housing 318 atareas 331. Areas 331 are depicted as a close fit between housing 318 andpiston 330, although other embodiments (not shown) may include a sealingelement between piston 330 and housing 318 at areas 331. Piston 330includes angled face 332 arranged to interface with corner 334 ofhousing 318. That is, piston 330 and housing 318 can be sealinglyengaged by contact between face 332 and corner 334. Valve spring 336urges piston 330 towards corner 334 of housing 318. Spring 336 isaxially retained by valve piston 338. Piston 338 axially retains innerportion 339 of spring 304.

Valve 300 further includes valve release spring 340 axially disposedbetween housing 404 and poppet 330. Snap-spring 304 controls an axialposition of valve piston 338 and compression of spring 336. In thenormal operating mode shown in FIG. 6, force of spring 340 is greaterthan force of spring 336 so that piston 330 is displaced in direction ofarrow 342 and away from corner 334. That is, snap-spring 304 displacesvalve piston 338 allowing displacement of piston 330 away from sealingengagement with housing 318 when valve 300 is in the open position.

Transmission 400 includes clutch 402, housing 404, and sealing plate406. Plate 406 is fixed to housing 318 and sealed to housing 302 withseal 408. Transmission 400 also includes shaft 410 sealed to housing 318with seals 412 and 414. Transmission clutch release spring 416 isaxially disposed between plate 406 and housing 302. Spring 416 aids inreleasing clutch pack 402. In some embodiments (not shown), overstrokespring 306 is not present in valve assembly 300 and release spring 416performs the function of overstroke spring 306. That is, spring 416contacts outer portion 312 of spring 304, urging spring 304 in directionof arrow 316. In the embodiments where overstroke spring 306 is notpresent, release spring 416 is the overstroke spring.

The following description is made with reference to FIG. 6A. FIG. 6A isa detail view of a portion of valve 300 circumferentially offset fromthe portion of valve 300 depicted in FIG. 6, shown in an open mode.Valve 300 includes open piston 330A and springs 336A and 340A disposedin a portion of housing 318 circumferentially offset from piston 330.

Valve 300 includes pressure ports 344, 346, and 348 disposed in housing318, and overstroke port 349 disposed in housing 302. Pressure ports 344and 346 may be connected to a pressure source. Ports 344 and 346 may beconnected to a transmission pump, for example. Valve 300 furtherincludes chambers 352, 354, and 355 in fluid communication withrespective ports 346, 348, and 349. Pressure chamber 352 is at leastpartially defined by housing 318 and piston 330A. Pressure chamber 354is at least partially defined by housing 318 and sealing piston 330.Pressure chamber 355 is at least partially defined by housing 302 andpiston 317. Chamber 356 is fluidly connected to a sump (not shown)through port 358 in housing 318 to drain fluid leaking past sealed areas331.

Operation of valve 300 will now be described. The following descriptionis made with reference to FIGS. 7 and 7A. FIG. 7 is a cross-sectionalview of valve 300 of FIG. 6 shown in a clutch apply mode. FIG. 7A is adetail view of a portion of valve 300 circumferentially offset from theportion of valve 300 depicted in FIG. 7, shown in a clutch apply mode.During normal operation as shown in FIGS. 6 and 6A, valve 300 is open.Pressurized hydraulic oil enters valve 300 through ports 344 and 346.Pressure in chamber 354 enters chamber 355 through port 349. Pressure inchamber 355 operates on overstroke piston 317 and snap spring 304. Solong as pressure in chamber 354 is below a predetermined limit, spring304 maintains the position shown in FIG. 7. Force of pressure inchambers 352 and 354 acting on respective pistons 330A and 330 urgesvalve piston 338 in direction of arrow 342. Pressure in chamber 354 andforce of spring 340 displace piston 330 so that ports 344 and 348 are influid communication. That is, flow introduced in port 344 flows pastpiston 330 to port 348 and chamber 354. Alternatively stated, flow froma transmission pump can proceed from port 344 through valve 300 and outport 348 to pressurize piston 302, urging piston 302 in direction ofarrow 342 engaging clutch pack 402.

The following description is made with reference to FIGS. 8 and 8A. FIG.8 is a cross-sectional view of valve 300 of FIG. 6 shown in a transitionmode. FIG. 8A is a detail view of a portion of valve 300circumferentially offset from the portion of valve 300 depicted in FIG.8, shown in a transition mode. Increased, overstroke pressure in chamber355 acting on snap spring 304 through pivot plate 315 and overstrokepiston 317 axially displaces outer portion 312 of spring 304 urgingoverstroke spring 306 in direction of arrow 342. It should be noted thatthe increased pressure is greater than a normal operating pressure forclutch 402. Axial deflection of inner portion 339 is restricted bycontact with valve piston 338 and sealing plate 406. Once the inputpressure exceeds a predetermined value, snap-spring 304 “snaps through”to the position shown in FIG. 8. The predetermined value can becontrolled in part by the force of overstroke spring 306.

As can be seen in FIG. 8, outer portion 312 of spring 304 has shifted indirection of arrow 342 displacing outer portion 310, and compressingoverstroke spring 306.

Axial force of pressure in chamber 354 acting on sealing piston 330 indirection of arrow 342 is greater than axial force of spring 336 actingin direction 316. Therefore, flow from port 344 to port 348 isunrestricted because angled face 132 remains displaced from corner 134.Similarly, pressure force acting on piston 330A in direction of arrow342 is greater than force of spring 336A acting in direction 316.

The following description is made with reference to FIGS. 9 and 9A. FIG.9 is a cross-sectional view of valve 300 of FIG. 6 shown in an idle-stopmode. FIG. 9A is a detail view of a portion of valve 300circumferentially offset from the portion of valve 300 depicted in FIG.9, shown in an idle-stop mode. Once pressure is removed from port 344and chamber 355, overstroke spring 306 urges snap-spring 304, pivotplate 315, and overstroke piston 317 in direction of arrow 316.Snap-spring 304 pulls valve piston 338 in direction 316. Axial force ofoverstroke spring 306 is greater than force of spring 340. Valve piston338 urges sealing piston 330 in direction of arrow 316. Remainingpressure in chambers 354 and 355 acting on area 366 of piston 130 urgespiston in direction of arrow 342 until force of spring 336 overcomes thepressure force. Therefore, springs 336 and 340, and area 366 can beadjusted to provide an appropriate pressure force for chamber 354.

Axial motion of spring 304 in direction of arrow 316 allows axial motionof piston 330 in direction of arrow 316 until face 332 contacts corner334 restricting flow of fluid between ports 344 and 148, closing valve300. Otherwise stated, input pressure acting on snap-spring 304displaces overstroke spring 306 and adjusts valve 300 into a closedposition when the input pressure is released. That is, valve spring 336displaces sealing piston 330 into sealing engagement with housing 318when valve 300 is in the closed position. Similarly, force of spring336A acting on piston 330A.

Therefore, pressure is maintained in chamber 354 and port 348 even whenthe pressure source (not shown) is not providing pressure. Force ofclutch release spring 416 can be adjusted to control position of piston302 relative to clutch pack 402. For example, piston 302 can keep clutchpack 402 engaged; piston 302 can be contacting clutch pack 402 withoutengaging the clutch; or piston 302 can be axially offset from clutchpack 402 by a small distance as shown in FIG. 9. Maintaining distancebetween piston 302 and clutch pack 402 assures clutch pack 402 isdisengaged, but the maintained pressure in chamber 354 advantageouslydecreases the clutch apply time.

The following description is made with reference to FIGS. 10 and 10A.FIG. 10 is a cross-sectional view of valve 300 of FIG. 6 shown in are-open mode. FIG. 10A is a detail view of a portion of valve 300circumferentially offset from the portion of valve 300 depicted in FIG.10, shown in a re-open mode. Valve 300 can be easily reset by the nextnormal pressure event. For example, after the stop event, pressureenters valve 300 through ports 344 and 346 axially displacing piston330A in direction of arrow 342. It should be noted that the pressure isless than the overstroke pressure required to close the valve. Piston330A displaces valve piston 338 in direction of arrow 342, allowingspring 340 to displace piston 330, restoring fluid communication betweenports 344 and 348 to engage and disengage the clutch pack. Axialdisplacement of outer portion 312 of spring 304 is restricted bycontract with inner diameter 310 of overstroke spring 306, so spring 304“snaps through” to the position shown in FIG. 10. Valve 300 remains inthis condition with fluid communication between ports 344 and 348 untilthe next high pressure event.

Of course, changes and modifications to the above examples of theinvention should be readily apparent to those having ordinary skill inthe art, without departing from the spirit or scope of the invention asclaimed. Although the invention is described by reference to specificpreferred and/or example embodiments, it is clear that variations can bemade without departing from the scope or spirit of the invention asclaimed.

What we claim is:
 1. A clutch for a vehicle transmission, comprising: atleast one clutch plate; a piston plate displaceable for engaging anddisengaging the at least one clutch plate to form a torque path throughthe at least one clutch plate; a chamber, partially formed by the pistonplate for receiving and discharging fluid to displace the piston plate;a snap-spring displaceable between first and second equilibriumpositions; a hydraulic valve, including: a housing with a first port forconnection to a source of pressurized hydraulic fluid and a second portin fluid communication with the chamber; a displaceable valve piston;and, a valve spring; wherein: in the first equilibrium position thesnap-spring holds the valve piston in an open position to enable fluidflow from the source of pressurized hydraulic fluid to the chamber viathe first and second ports; and, in the second equilibrium position thesnap-spring holds the valve piston in a closed position compressing thevalve spring; an overstroke spring, wherein moving from the firstequilibrium position to the second equilibrium position comprisesdisplacing the overstroke spring; and an overstroke piston, wherein theoverstroke piston displaces the overstroke spring in response tohydraulic fluid pressure greater than a normal operating pressure forthe clutch.
 2. The clutch of claim 1 wherein the overstroke spring is aclutch release spring.
 3. The clutch of claim 1 further comprising adisplaceable sealing piston, wherein, during an engine idle mode,hydraulic pressure is withdrawn from the first port and the valve springdisplaces the sealing piston to block the second port and trap hydraulicfluid in the chamber.
 4. The clutch of claim 3 further comprising avalve release spring, wherein the valve release spring displaces thesealing piston when the snap-spring is in the first equilibriumposition.
 5. The clutch of claim 4 wherein the valve spring generates anapply force, the valve release spring generates a release force, and theapply force is greater than the release force when the valve spring iscompressed by the valve piston.
 6. The clutch of claim 1 wherein: theclutch includes a chamber for receiving and discharging fluid todisplace the piston plate; and, the overstroke piston displaces theoverstroke spring in response to hydraulic pressure in the chambergreater than a normal operating pressure for the chamber.
 7. The clutchof claim 1 further comprising an open piston, wherein moving from thesecond equilibrium position to the first equilibrium position comprisesdisplacing the open piston.
 8. The clutch of claim 7 wherein thepressurized hydraulic fluid displaces the open piston, and wherein apressure of the pressurized hydraulic fluid for displacing the openpiston is less than or equal to a normal operating pressure for theclutch.